Safe and Arm (S&A) devices are used to prevent an explosive device's main charge from inadvertently detonating, e.g., while stored or handled. These devices allow the explosive to detonate when desired or intended, e.g., when delivered to a target.
Two military specifications set forth standards that relate to fusing: Mil-Std-1316 for fuses; Mil-Std-1455 for dispensed projectiles and submunitions. These specifications include the following standards:                1. While in safe mode, the S&A device must not allow more than 1 in 1 million detonation transfers from primary to secondary explosive.        2. A submunition's S&A device should not allow packaging of the device in the dispenser in armed mode.        3. There must be two independent arming environments sensed by the S&A device that allow the device to go from safe mode to armed mode.        4. The two environments must occur after launch and after safe separation has occurred.        5. In the event the arming environments are taken back away, the S&A device must return to safe mode.        6. In armed mode, the S&A device should allow transfer from primary to secondary explosive if the explosive train is initiated. For further details, the interested reader is referred to the applicable standards, such as Mil-Std-1316 and Mil-Std-1455, the entire contents of which are incorporated herein by reference.        
Three known types of S&A devices make use of sliders, rotors, and shutters. A physical barrier (e.g., metal) separates a primary explosive from a secondary explosive in an explosive device. These devices can take up more than three times the amount of space as the explosive material's transfer diameter. The transfer diameter is the minimum diameter needed in intimate contact between primary explosive and secondary explosive to achieve a reliable detonation transfer from primary explosive to secondary explosive. For example, the transfer diameter for typical explosives is 0.11 inches. A rotor that eccentrically turns a primary material to be inline with the secondary would need to be about 0.375 inches in diameter to swing a 0.125 inch diameter in line.
A set back and spin S&A device can be used in gun rounds. For example, an artillery gun round S&A can use set back as environment 1. The set back environment pulls a pin out of a plate mounted eccentrically on a shaft. Removal of the set back pin allows the plate to rotate about the shaft. The gun round is spun up by rifling in its barrel while the set back is present, so the eccentric plate can swing a primary in line with a secondary to arm the device.
An example of an artillery gun round fuse containing the S&A device is 2.5 inches in diameter. Unfortunately, if you scale down these S&A devices to a smaller diameter, they no longer work. The environments (accelerations) they use are still there, but the mass of the tiny pieces are so small they may not reliably overcome friction and springs to enable the armed condition. Also, the transfer diameter is scaled below a level where it will function reliably. The M758 fuse used with the 25 mm M242 gun is an example of an S&A device that works correctly for its specific size, but may not scale to operation at a smaller size.